JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<head>
    <title>Test</title>
</head>
<body>
    <div id="wrapper">
        <div id="main">
            <form>
                <textarea id="input" name="input" rows="30" cols="30"></textarea>
            </form>
            <div id="output"></div>
        </div>
    </div>
</body>
</html>

CSS

#main{
    position:relative;
    display:block;
    height:auto;
    overflow:auto;
}

form{
    position:relative;
    display:block;
    float:left;
    margin-right:1em;
}

#output{
    position:relative;
    display:block;
    height:100%;
    border:1px solid #000;
    float:left;
    width:20em;
    min-height:20em;
}

JavaScript

'use strict';
    var inputForm = document.getElementById("input");
    inputForm.onkeypress = function () {
        var output = document.getElementById("output");
        output.style.backgroundColor = '#000';
    };